The leader of the Muslim community is not but he who acts according to the Quran, exercises justice, accepts the truth, and devotes himself to [the cause of] Allah.
After the people of Kufa sent letters to Imam Hussain (as) inviting him to come to their city and expressed their readiness to accept his leadership, the Imam (as) sent them a reply with his deputy, Muslim ibn Aqil.
In his letter, enumerating the conditions that the leader of the Muslim community should have, Imam Hussain (as) wrote,
The leader of the Muslim community is not but he who acts according to the Quran, exercises justice, accepts the truth, and devotes himself to[the cause of] Allah. (Mufid, al-Irshad, pp. 380-381)
According to the grandson of Prophet Muhammad (pbuh&hf) and master of the youths in Paradise, these are the criteria that the leader of the Muslim community should meet, otherwise he would not be a legitimate leader. The Umayyad and Abbasid dynasties, in order to maintain their illegitimate rule, fabricated some hadiths that proposed the leader of the Muslim community should be obeyed no matter how evil he is. Even today, the scholars of some Muslim schools of thought endorse such hadiths and claim that the leader of the Muslim community should be obeyed even if he consumes alcohol or commits other sins. According to them, it would be a sin to oppose the ruler even if he is the worst of people or has assumed power by force.
The question remains that if such hadiths were true, why the closest relative of the Prophet (pbuh&hf) who was the most knowledgeable of the Islamic faith and the most devoted among Muslims, staged his historical revolt against the incumbent caliph of his time.